It is known that a field unit such as a positioner, a response synchro, or a limit switch is frequently attached to or installed on an electrical or pneumatic servo/actuator for positioning a valve in accordance with DIN IEC 60534-6.
The mounting of the field unit must encompass essentially two tasks: First, a connection must be established between the field unit and the actuator; second, the linkage—usually mechanical—by which the position of the actuator is conveyed to the field unit must be connected and adjusted.
This procedure does not usually cause any problems when performed at the factory of the manufacturer. When a unit is installed in the field, however, considerable difficulties can arise.
Depending on the weather conditions and on the accessibility of the drive unit, the following aspects of the mounting process can be disadvantageous: The simultaneous mounting of the field unit and the connection of the displacement indicator linkage—usually done by introducing a pin into a slot—requires two hands and depends on the exact positioning of the field unit. This can lead to an incorrect connection of the displacement indicator linkage, which in turn causes the actuator to operate incorrectly.
The housing of the field unit, furthermore, must be opened to allow the manual positioning of the displacement indicator linkage. This leads to the disadvantage that dirt and moisture can intrude into the interior of the field unit.
In addition, the manual positioning of the field unit also means that expensive measures must be taken to design the interior of the unit so that it can be adjusted properly. Providing a mechanical access hole, for example, and a corresponding opening in the circuit board for the electronic components is associated with high manufacturing costs.
The manual operations which must be performed when mounting the field unit on the actuator, namely, the positioning and adjustment of the components, also present the risk of injury.
To minimize the problems mentioned above associated with the mounting of a field unit on an actuator, various elaborations of the interface between the field unit and the actuator are known. For example, in the case of the TZID-C positioner developed by ABB, the engagement of a pin in a slot is replaced by the engagement of a spring-loaded cone in a slot. Although this does make mounting easier, most of the disadvantages mentioned above, such as the need to open the field unit, the two-handed procedures, etc., remain unaffected.
Another improvement in the ease of mounting is provided by the Samson 3730 family of positioners. Here the pin-in-slot connection is replaced by a spring-loaded pin, which rolls along a plane. It is therefore no longer necessary to position the pin accurately in the slot within a narrow range of tolerances. Instead, the linking lever can be pushed well away from the contact position during the mounting procedure. Although this again makes it easier to install the unit, the disadvantages of two-handed manipulation and of the need to open the field unit, with all the associated design disadvantages, remain unaffected.